Method for determination of combined oxygen in liquid alkali metals



y 30, 1957 J. w MAUSTELLER ETAL METHOD FOR DETERMINATION OF COMBINEDOXYGEN IN LIQUID ALKALI METALS Filed June 4, 1962 INEIZ'I' GAS METERLIQUID ALKALI METAL CONTAINING COMBINED OXYGEN LIQUID METAL INVENTORS.qb/m Wnsozv Maw-ream,

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their a 7- roea/s Y2 United States Patent 3,323,950 METHOD FORDETERMINATION OF COMBINED OXYGEN IN LIQUID ALKALI METALS John WilsonMaustellcr, Evans City, and Sheridan J.

Rodgers, Ellwood City, Pa., assignors, by mesne assignments, to MineSafety Appliances Company, Pittsburgh,

Pa, a corporation of Pennsylvania Filed June 4. 1962. Ser. No. 199,953 5Claims. (Cl. 324-65) This invention relates to the quantitativedetermination of oxygen in liquid alkali metals, and more particularlyto its determination indirectly by measuring the electrical resistanceof an oxygen gathering metal surface exposed to the liquid alkali metal.

Liquid alkali metals have been used for a great variety of purposes andare widely used as heat transfer media in closed loop heat transfersystems. The determination of oxygen content, the oxygen being presentin combined oxide form with the liquid alkali metal, is necessary toassess the corrosion potential and determine dissolution rates ofstructural materials. A number of methods of determining oxygen areknown, but they require the removal of a sample of liquid metal forsubsequent chemical manipulation. Such methods are cumbersome and timeconsuming, require a high degree of skill, and because the liquid alkalimetals are pyrophoric, are not Without hazard.

It is therefore an object of this invention to provide a method ofdetermining the oxygen concentration in liquid alkali metals in situ.Another object is to provide such a method that is simple, easilyperformed, and utilizes an easily measurable electrical quantity. Yetanother object is to provide simple and inexpensive apparatus forperforming the method of the foregoing objects.

Other objects will appear from the following description. I

This invention is based on the fact that alkali metal oxides will reactwith a second metal to form an oxide of the second metal if the freeenergy of formation of the oxide of the second metal is .greater thanthat of the alkali metal oxide. Such a second metal and electrodesformed therefrom are herein referred to as oxygen gathering metals oroxygen gathering electrodes. We have discovered that the electricalresistance of an oxygen gathering electrode contacted with a liquidalkali metal increases linearly with a rate of increase proportional tothe oxygen content of the liquid metal, and that the electricalresistance can simply and conveniently be measured to indicate theoxygen content of a liquid alkali metal.

In the practice of this invention, an oxygen gathering electrode isimmersed in a body of liquid alkali metal to be tested for oxygencontent and the electrode-liquid alkali metal interface is included in aresistance measuring circuit. As the oxygen gathering electrode reactswith the oxygen contained in the liquid metal a film of oxygen gatheringmetal oxide forms thereon increasing the electrical resistance betweenthe electrode and the liquid alkali metal since the metal oxides aremuch poorer conductors of electricity than the metals. The electricalresistance increases linearly at a rate proportional to the oxygenconcentration in the liquid alkali metal until the entire immersedsurface of the electrode is coated with an oxide film. At this point therate of resistance change decreases markedly, the rate apparently beingdependent on the rate of diffusion of oxygen gathering metal oxide intothe body of oxygen gathering metal. This break point is readilydeterminable in use because it is indicated by an abrupt change of slopein a plot of resistance against time, such as can be provided byconventional recording apparatus. Such plots may be used to predeterminethe break point, or oxygen gathering capacity, for an electrode of anygiven metal and configuration. The initial linear rate of resistanceincrease is a measure of oxygen content and is readily determined byperiodic measurement of the resistance and calculation of the rate ofchange from two or more of such measurements. It will be apparent tothose skilled in the art that the rate may be recorded or read outdirectly with suitable recorders and simple calculators.

The rate of resistance change for a given oxygen concentration will bedifferent for each different combination of oxygen gathering metal andliquid alkali metal to be analyzed, so calibration for the particularcombination used is necessary. Suitably, the analyzer is calibrated bydetermining the rate of resistance change with a given electrode and agiven liquid alkali metal by comparison with chemical analysis of theliquid metal containing varying amounts of oxygen. This data isconveniently correlated in a table or family of curves with parametersof oxygen concentration and rate of resistance change.

The accompanying drawing is a schematic representation of an oxygenanalyzer embodying this invention. Liquid alkali metal 2 is contained bymetal vessel 4, which is conveniently an integral part of the system inwhich the liquid metal is being used. An oxygen gathering electrode 6 isimmersed in the liquid metal and is supported by electrical conductingrod 8 which is insulated from the vessel by a gas tight insulatingsleeve 10. A DC. current source 12 and ohmmeter 14 are electricallyconnected to rod 8 and the vessel 4 completing a circuit including thevessel, the liquid metal and the electrode 6. As the oxide film forms onthe electrode, imposing a high resistance in the circuit between theelectrode and the liquid alkali metal the resistance is indicated by theohmmeter. The rate of resistance change is determined from periodicresistance measurements to indicate oxygen concentration as heretoforedescribed.

To provide for adjustment and removal and replacement of the electrode,rod 8 is slideably engaged by insulating sleeve 10. The sleeve ismounted in a detachable tubular section 16, separated from the system byvalve 18, suitably a gate valve, which when closed isolates the liquidmetal system and when opened can pass the electrode. To introduce theelectrode, rod 8 is positioned so that the electrode is within thetubular section 16. The tubular section is attached to the system atvalve 18 forming an air lock which is flushed with inert gas throughconduits 20 and 22 to prevent atmospheric contamination of the liquidmetal system. The valve is then opened and the electrode is lowered intothe liquid metal.

When increased sensitivity is required, that is, a larger resistancechange for a given concentration of oxygen, the circuit may be completedthrough a second electrode immersed in the liquid metal rather than byconnection to the vessel. The resistance of the oxide film is thusimposed in the circuit at both electrode surfaces. When two electrodesare used they may be formed of the same or different oxygen gatheringmetals.

The resistance of the circuit will vary if the area of electrode surfacein contact with the liquid metal changes, so it is necessary for precisemeasurements to maintain a constant level of electrode immersion. Invessels wherein the liquid metal level changes substantially, theelectrode may be raised or lowered in coordination with the levelchanges. Preferably, the electrode is totally immersed in the liquidalkali metal below the lowest anticipated liquid metal level so thatadjustment for level changes is not necessary; similarly totallyimmersed electrodes may be used for analyzing flowing stream in pipes orconduits. When the electrode is totally immersed, the conductingelectrode support rod must be insulated from the liquid alkali metal, asby coating with an insulating refractory. Conveniently, the electrodesupport may be formed from an oxygen gathering metal which has beeninactivated by preforming an oxide coating on it.

Since the analysis is dependent on a chemical reaction, the rate ofresistance increase is also dependent on temperature. Although it ispreferred to carry out the analysis at a substantially constanttemperature, generally satisfactory precision is obtained whentemperature variation cannot be avoided by considering the analysis tobe made at an average temperature and averaging fluctuation of rate ofresistance change over the period of analysis.

Table 1 lists the free energies of formation of oxide of metals whichcan be used as electrodes and the alkali metals. The listing is arrangedwith the most stable oxide at the top, progressing to the least stableoxide at the bottom.

A metal whose oxide is more stable than a given alkali metal oxide is anoxygen .gathering metal in that system and is suitable for use as anelectrode material. The relative stability of oxides in some caseschanges with temperature, according to well known thermodynamicsprinciple, so some oxygen gathering metals are operative only overlimited temperature ranges for determining oxygen in a given alkalimetal. For example, although beryllium is not suitable for determiningoxygen in lithium at low temperatures, such as 300 K., because berylliumoxide is less stable than lithium oxide, it is suitable at highertemperatures, e.g. 750 K.

From the table for example, it can be seen that at 750 K. only thorium,magnesium and beryllium are suitable electrode materials for use inanalyzing lithium for oxygen. These three metals as well as yttrium,aluminum, hafnium, cerium, zirconium, titanium and niobium can be usedto analyze sodium. All the foregoing plus chromium and zinc can be usedto analyze potassium.

must be higher than the liquid metal temperature, so that the oxide filmcan form on the solid state electrode.

Preferred electrode materials for analyzing alkali metals other thanlithium are the high melting zirconium and hafnium. Aluminum, because ofits cheapness and availability, is especially preferred for analysis ofalkali metals other than lithium at temperatures below about 1000 F.

The following example of analysis performed using a two active electrodecircuit illustrates this invention. Two zirconium electrodes, supportedby insulated rods, Were partially immersed in liquid sodium containing0.005% oxygen at 600 F., as determined by chemical analysis. Theresistance increased linearly for about 2 hours at a rate of 1.6 ohmsper hour, at which time the rate of resistance increase decreasedsharply showing the effective measuring capacity of the electrode hadbeen used. Two zirconium electrodes of the same size were immersed tothe same depth in liquid sodium containing 0.35% oxygen at 600 F. asdetermined by chemical analysis. The resistance increased linearly forabout one hour at a rate of 3.7 ohms per hour.

Although in the foregoing description and examples the electricalresistance increase has been measured directly, it will be apparent tothe artisan that it can be measured indirectly equally as well, as, forexample, by measuring the change in voltage with a constant current.

According to the patent statutes we have explained the principle of ourinvention and have illustrated and described what we now believe to beits best embodiments. However, we desire to have it understood that,within the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

We claim:

1. A method of determining the concentration of combined oxygen inliquid alkali metals comprising contacting the liquid alkali metal to betested with an electrode of a metal that forms an oxide more stable thanthe oxide of the alkali metal and measuring the substantially linearrate of electrical resistance increase of said electrode occurringresponsive to the presence of combined oxygen in the liquid alkalimetal.

2. A method according to claim 1 in which aconstant temperature ismaintained.

3. A method according to claim 1 in which the electrode is zirconium.

4. A method according to claim 1 in which the electrode is hafnium.

5. A method according to claim 1 in which the electrode is aluminum.

References Cited UNITED STATES PATENTS 2,374,088 4/1945 Fontana et al.3243O 2,525,754 10/1950 Albrecht 32430 2,593,878 4/1952 Haines et al.32430 RUDOLPH V. ROLINEC, Primary Examiner.

WALTER L. CARLSON, Examiner.

C. A. S. HAMRICK, C. F. ROBERTS,

Assistant Examiners.

1. A METHOD OF DETERMINING THE CONCENTRATION OF COMBINED OXYGEN INLIQUID ALKALI METALS COMPRISING CONTACTING THE LIQUID ALKALI METAL TO BETESTED WITH AN ELECTRODE OF A METAL THAT FORMS AN OXIDE MORE STABLE THANTHE OXIDE OF THE ALKALI METAL AND MEASURING THE SUBSTANTIALLY LINEARRATE OF ELECTRICAL RESISTANCE INCREASE OF SAID ELEC-